1. Field of the Invention
This invention relates to apparatuses for controlling the exchange of air through doorways or other openings to refrigerated spaces, and more particularly to an improved conditioned air vestibule for use at a doorway of a refrigerated or cold storage room.
2. Description of the Prior Art
Doors provide access to cold storage rooms from anterooms or loading docks or other adjacent spaces for material handling vehicles and pedestrian traffic. Traffic through the doors is frequently heavy particularly at peak periods of the day so that the doorways are necessarily open at least a substantial portion of the time and many are kept open continuously during such peak traffic periods. Such open doorways present problems both with regard to operation and maintenance of refrigeration equipment and with regard to the productivity and safety of the facility.
As is recognized, an open doorway to a refrigerated space permits the heavier refrigerated air to flow out of the refrigerated space through the lower half of the opening and an equal mass of warm humid air to flow inward through the upper half. In this air exchange, warm air entering the refrigerated space is referred to in the industry as infiltration air, and cold air escaping is sometimes referred to as exfiltration air.
When a warm air mass encounters a cold air mass, precipitation commonly occurs, the eventuality of this phenomenon depending upon conditions of the two air masses relative to one another. The form of precipitation, i.e., water droplets or airborne ice crystals, depends upon the temperature of the mixture.
The warm and cold side conditions at the entrance to subfreezing cold storage rooms or freezer rooms are generally in the precipitation range relative to each other, at least during mild and warmer weather, and almost always in the warmer climates. As warm air enters through the top of a freezer room door, precipitation in the form of airborne ice crystals is visible as haze while visible fog frequently appears outside the door as cold air escapes from the bottom of the door and mixes with the warm humid outside air. Warm side fog can obstruct the vision of personnel, including vehicle operators, working in the area. In addition, the chilled fog-laden air frequently causes wet slippery floors in the vicinity of the doorway with consequent hazards not only to personnel but also to equipment and material.
Precipitation from infiltration air is generally found to be even more objectionable than fog outside the door. The airborne ice crystals result in frost or snow accumulation on ceilings, walls, and freezer room appurtenances as well as on products stored in the room. Such frost frequently grows to many inches in thickness and can result in snow droppings which cause icy floors and present extremely slippery and hazardous conditions for forklift trucks. Further, the airborne ice crystals may be drawn into the refrigeration equipment and produce premature clogging of the coils, as compared with normal evaporator coil icing, thereby reducing the refrigeration effect and adding coil defrosting burden. The result is a substantial reduction in refrigeration efficiency and may require installation of additional evaporator coils or oversized refrigeration equipment.
Many attempts have been made to reduce the air exchange at open refrigerated warehouse doors. One approach has been to employ an air curtain across the door, with the forced flow of relatively high velocity air across the opening serving to restrict the normal air exchange resulting from the temperature differential. It is also known to condition air used in such air doors by heating the air employed in the air curtain to reduce precipitation both inside and outside the refrigerated space. Examples of such devices may be found, for example, in U.S. Pat. Nos. 3,218,952, 3,817,160, 4,516,482 and U.S. Pat. No. Des. 264,561.
A relatively short conditioned air vestibule having two spaced air curtain doors employing conditioned air in the air curtain is illustrated in U.S. Pat. No. Des. 140,200. Such devices, while effective in reducing precipitation both inside and outside a refrigerated warehouse door, they don't eliminate such precipitation.
Physical barriers, particularly the well-known strip doors, are also widely used to restrict the flow of air through an open refrigerated warehouse door. Such strip doors employ transparent vinyl strips which enable personnel and vehicles to push through, with the strips quickly falling back into place to act as an air flow barrier when the obstruction has cleared the door.
Another known system for controlling precipitation from infiltration or exfiltration air employs a step-down room at the door, with the step-down room having a physical barrier such as a strip door or rigid push-through door at each end for restricting air exchange. The air inside such step-down rooms is heated to a non-fogging or non-frost producing level and to prevent airborne crystal formation in the refrigerated room as a result of air infiltration. This level of heat is normally found sufficient to prevent fog formation as a result of air exfiltration from the step-down room. The known step-down rooms are of sufficient size to permit material handling vehicles to enter one end and the door to close behind it before reaching and pushing through the door at the other end. Such arrangements are therefore costly both because they occupy substantial floor space and because of the relatively large volume of heated air required.
Large step-down rooms also have generally been considered objectionable in that their tunnel configuration tends to restrict the vision of forklift operators and therefore can present a safety hazard. For this reason, it has been common practice to provide two step-down rooms to enable one way traffic entering and leaving the cold storage room.
The use of push through strip doors is also objectionable in that the strips tend to become less transparent with use and may present an obstruction to vision. Further, frost or fog condensation on the strip surfaces not only obstruct vision, but the wet, cold surfaces are generally considered objectionable by personnel passing through the door. The relatively heavy plastic strips can also drag lightweight items such as empty cartons from material handling equipment.